Aerosol optical particle counters of the prior art typically utilize either a mass flow sensor or a pressure sensor, monitoring pressure drop across a restriction, to establish the flow rate within the particle counter. Because these devices are small, inexpensive and readily available, they are extensively used in industry.
By way of example, the prior art is familiar with particle sensors and associated airflow systems using a centrifugal blower, as described in U.S. Pat. No. 5,515,164. FIG. 1 of the '164 patent illustrates a representative prior art particle sensor 10 and airflow system 12 in this category. Particle sensor 10 has a sensing cavity 10a; and airflow system 12 has a blower cavity 12a. Airflow system 12 draws air from sensing cavity 10a such that air and particulates 14, from room 16, are drawn into sensing cavity 10a through hose 18 and airflow tube 20. Airflow tube 20 is tapered along a single axis such that air and particulates 14 from room 16 are forced into a narrow uniaxial flow 22 within sensing cavity 10a.
Particulates within flow 22 are detected through known techniques. Specifically, laser 24 generates a laser beam 26 that orthogonally illuminates flow 22, creating scattered energy 26a which is functionally dependent upon particulates within flow 22. Detector 28 detects scattered energy 26a; and detector signals from detector 28 are analyzed by particle counting electronics 30 to quantify the number of particles within the flow (for purposes of illustration, detector 28 is shown collecting scattered energy 26a in the same plane as beam 26; while in reality detector 28 typically views scatter 26a orthogonally to both beam 26 and flow 22).
Airflow system 12 includes a centrifugal blower 32 powered by motor 34. Centrifugal blower 32 draws air from within sensing cavity 10a and directly through its center 32a coaligned with flow 22. Exhaust 14a from blower 32 primarily exits blower cavity 12a as exhaust 14b through filter 36; except that a portion of exhaust 14a, denoted as exhaust 14c, is captured by tube 38 and routed to flow sensor 40 to assess the flow rate through blower 32. Blower controller 42 receives electrical signals indicative of flow rate from sensor 40, through signal line 44; and commands motor 34, through signal line 46, to drive blower 32 to a preset speed according to the flow rate. Blower controller 42 thus operates in feedback control of blower 32 via flow rate sensor 40 and motor 34.
It is desirable ill the field of particle sensing to determine the number of particulates for an exact volumetric flow rate (i.e., one cubic foot per minute, or "CFM") as sampled from the ambient environment within room 16. By way of example, particle sensors in "clean rooms," known in the art, assume that contamination levels are measured relative to a volume of air, and not relative to mass, since the mass of a given volume of air can vary by over 20% due to elevational changes alone. Weather fronts induce additional mass-per-volume differentials of up to 3%.
Therefore, one problem in the particle sensor and airflow system 10, 12 of FIG. 1 is that actual volumetric flow rate is calibrated for a given pressure, typically at "sea level," such that particle counting at other locales does not reflect a correct volumetric flow rate. Manual adjustments can be made to compensate for these errors, except that continuous manual adjustment is needed to ensure accuracy over changing environmental (e.g., elevation and barometric pressure) conditions.
Another problem in airflow system 12, FIG. 1, is that centrifugal blower 32 is arranged co-axially about flow 22 because of the physical operation of blower 32: by definition, centrifugal fans draw air through the center of the fan. Accordingly, airflow system 12 has an elongated form along the axis of flow 22, reducing the compactness of airflow system 12 as desired, for example, in hand-held particle sensing applications. Accordingly, to improve compactness, airflow tube 20 is typically made shorter than desired for optimal performance.
Yet another problem with airflow system 12, FIG. 1, is that centrifugal blower 32 is limited in the amount of pressure differential it can handle between sensing cavity 10a and blower cavity 12a, thereby restricting the capability of particle sensing to certain environments. This limitation in operating within certain pressure differentials also limits the amount of filtering which can exist after blower 32 to reduce contamination. More particularly, filter 36 cannot, generally, be a high performance contamination filter because such filters cause additional pressure differential. Prior art airflow systems 12 thus typically utilize a crude filter 36, increasing the likelihood of contamination into sensing cavity 10a or room 14.
It is, accordingly, one object of the invention to provide a particle sensor and airflow system that reduces or eliminates the above-described problems in the prior art Yet another object of the invention is to provide methods of particle sensing with respect to absolute volumetric flow rates regardless of the particle sensor locale. Still another object of the invention is to provide a compact particle sensing system with reduced internal contamination and with reduced external contamination to the environment under measure. These and other objects will be apparent in the description that follows.